1. Field of the Invention
The present invention relates to magnetic recording media and magnetic recording apparatus in which the recording media are equipped, in particular, to magnetic recording media having high recording density and magnetic recording apparatus such as hard disk drives in which the high-density recording media are equipped.
2. Description of the Related Art
Hard disk drives (HDDs) have been expanding their application scope from the first computer related application to various other applications, such as home video recorder and car carrying navigation system applications as magnetic recording systems for recording and reproducing information. The expansion is due to their advantage of high data access speed and high data storage reliability and so on in addition to their high recording capacity performance with low cost. Requirements for HDDs having larger recording capacity have been increased with the expansion of the HDD application scope. Replying to the requirements, large capacity recording technology has been advanced by increasing recording density of the magnetic recording media.
With increasing recording density of the magnetic recording media of HDDs, the recording bit sizes and the diameters for the magnetization reversal units became very small. As the result, decreasing phenomena of the recorded signal magnetization and the recording and reproducing performance caused by thermal fluctuation were obviously appeared for the very small magnetization reversal units. Furthermore, noise signals which appear at boundary regions between recording bits became large as a result of decreasing recording bits to a very small size, and the noise gave a large influences upon the signal to noise ratio. For the purpose of attaining further high recording density, it is required to obtain thermal stability of the recorded signal magnetization at one hand, and is required to attain low noise characteristics at high recording density at the other hand.
In order to decrease magnetic recording medium noise, size of magnetic crystalline grains constructing recording-layer have been made smaller up to now. For example, magnetic crystalline grains of Co—Cr magnetic-layer of widely used magnetic recording media have been made small by adding small amount of Ta or B (refer to Japanese Patent Laid-open Applications Nos. HEI 11-154321 and 2003-338029), and by precipitating nonmagnetic Cr by heat treating at appropriate temperature (refer to Japanese Patent Laid-open Applications Nos. HEI 3-235218, and HEI 6-259764). Recently, a method for obtaining magnetic recording layer having so called granular structure obtained by adding oxides such as SiOx to the magnetic layer was applied. In the granular structured magnetic layer, nonmagnetic grain boundary material encloses magnetic crystalline grains (refer to Japanese Patent Laid-open Applications Nos. HEI 10-92637, and 2001-56922).
Since these methods are not sufficient for making the magnetic crystalline grains of the magnetic layers smaller and are not sufficient for separating each grains magnetically, a method of using under-layers having smaller grain diameters was applied to obtain smaller magnetic crystalline grains (refer to Japanese Patent Laid-open Applications Nos. HEI 10-92637, and 2000-200410 for example) in addition to the methods described above.
As the crystalline grains in the under-layer are made small, however, there occurred a problem of decreasing the crystalline grain orientation degree in the under-layer. The decrease of under-layer crystalline orientation degree influences upon the magnetic crystalline grains as a decrease of crystalline orientation degree, and that results in decreases of overwrite characteristics and signal to noise ratio of the recording and reproducing characteristics.